This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Even with plenoptic cameras, blur in some out-of-focus parts of an image still remain (and it is not possible to focus on in this places) due to the fact that these out-of-focus parts correspond to elements in the object space that are far from the focalization distance. In conventional photography, this blur is also named bokeh. As specified in Wikipedia for bokeh: “In out-of-focus areas, each point of light becomes an image of the aperture, generally a more or less round disc. Depending how a lens is corrected for spherical aberration, the disc may be uniformly illuminated, brighter near the edge, or brighter near the center. Lenses that are poorly corrected for spherical aberration will show one kind of disc for out-of-focus points in front of the plane of focus, and a different kind for points behind. This may actually be desirable, as blur circles that are dimmer near the edges produce less-defined shapes which blend smoothly with the surrounding image. The shape of the aperture has an influence on the subjective quality of bokeh as well. For conventional lens designs (with bladed apertures), when a lens is stopped down smaller than its maximum aperture size (minimum f-number), out-of-focus points are blurred into the polygonal shape formed by the aperture blades.”
It should be noted that “high quality” bokeh is viewed by most photographers as out of focus areas that are smooth rather than harsh. Moreover, in case of a color image, the quality of bokeh is linked to the homogeneity of colors in out-of-focus part of the image (i.e. without color artifacts). More details on Bokeh are described in the article entitled “A Technical View of Bokeh” by Harold M. Merklinger in Photo Techniques, May/June 1997, or in the technical note: “Depth of Field and Bokeh” by H. H. Nasse from the Camera Lens Division of the Zeiss company.
The obtaining of color images from a plenoptic camera (as the one depicted in FIG. 1) generally involves a color demosaicing process that consists in determining, for each pixel of the image sensor, the two color channel representations that have not been recorded by the pixel (i.e. the missing colors). Indeed, as for traditional digital cameras, a plenoptic camera comprises a color filter array (noted CFA) placed onto the image sensor so that each pixel only samples one of the three primary color values. Such Color Filter Array is usually a Bayer type CFA which is the repetition of a Bayer pattern that can be represented as a matrix
  A  =            (              a        ij            )                      0        ≤        i        ≤        1                    0        ≤        j        ≤                  1          ′                    with 2 lines and 2 column, where a00=a11=G (for Green), a01=R (for Red), and a10=B (for Blue). For example, the FIG. 18B of document US 2014/0146201 presents an image sensor recovered by a Color Filter array with the repetition of such Bayer pattern. Another Bayer pattern is represented by a matrix
  B  =            (              b        ij            )                      0        ≤        i        ≤        1                    0        ≤        j        ≤                  1          ′                    with 2 lines and 2 column, where b01=b10=G (for Green), b00=R (for Red), and b11=B (for Blue).
However, for a plenoptic camera, with a Color Filter array comprising the repetition of a Bayer pattern represented by a matrix of dimension M×M, and in the case that the size of the diameter of the micro-images (noted asp) is equal to k×M (i.e. k times M), where k is an integer, then it is not necessary to apply a color demosaicing process when obtaining a refocused image. Indeed, as detailed in FIG. 3(b), in this configuration, the sub-apertures images obtained from such plenoptic camera are mono-chromatic, and as the refocusing process comprises the adding of these sub-apertures images, there is no need to perform a demosaicing. The refocusing can be viewed as a demosaicing less operation.
However, this architecture for a plenoptic camera has a drawback: the quality of bokeh in refocused images obtained from light field data acquired by such plenoptic camera is bad (due to the presence of color artifacts), especially during a refocusing process on other part of the image. These color artifacts are very difficult to correct, and occur each time some objects are observed out-of-focus by the main lens of a plenoptic camera. More precisely, a bokeh corresponding to a white circle form of a white light source point could display the Bayer pattern when refocusing, instead of keeping the same homogeneous color (see FIGS. 4(a) and 4(b) of the present document). Another issue induced by such architecture is that when a change of viewpoint in images (especially in the extreme viewpoints) is done, as the subaperture images are monochromatic, it will not be possible to obtain a good demosaiced image.
For correcting such issue, one solution consists in applying a color filter array (based on a Bayer pattern) directly on the micro-lens of a plenoptic camera instead of positioning it on the image sensor itself. Such technique is briefly described in FIG. 3 of the present document, or in the FIG. 2 of document US 2015/0215593. However, there is a need for a solution that does not need to change the position of a CFA from the image sensor to the micro-lens array.
One skilled in the art, trying to keep the CFA positioned on the image sensor, would have breakdown the regularity of the CFA by using random patterns as mentioned in paragraph [0128] of document US 2014/0146201.
However, there is a need to determine which pattern configuration is well fitted for solving the previous mentioned problem.